(1) Field of the Invention
The present invention relates to a carrier of a developer. More particularly, the present invention relates to a carrier of a developer capable of preventing so-called carrier dragging and providing an image having an excellent quality without occurrence of such troubles as fogging, letter thinning and rear end blurring at the developing step and a process for the preparation thereof.
Furthermore, the present invention relates to a developing process in which a two-component type developer comprising this carrier is advantageously used.
Incidentally, by the term "carrier dragging" is meant an undesirable phenomenon that in a two-component type developer, a carrier is transferred to a photosensitive material together with a toner and development is carried out in this state. By the term "fogging" is meant the phenomenon of transfer of a toner and the like to a background portion of a copying sheet. Furthermore, by the term "letter thinning" is meant a phenomenon that a letter or line is thinly developed, and by the term "rear end blurring" is meant blurring of the rear end of an image area on a copying sheet.
(2) Description of the Related Art
A two-component type developer comprising a magnetic carrier and a toner is widely used in the field of commercial electrophotosensitive material, and at the development of a charged image, a magnetic brush of this developer is formed on a developing sleeve having magnetic poles disposed in the interior thereof, and this magnetic brush is brought into sliding contact with a photosensitive material having the charged image formed thereon to form a toner image.
It is known that a ferrite carrier can be used as the magnetic carrier. For example, Japanese Unexamined Patent Publication No. 60-170863 teaches that a ferrite carrier having a resistivity lower than 5.times.10.sup.7 .OMEGA.-cm and a particle size of 50 to 120 .mu.m is used as the magnetic carrier of the two-component type developer, and that by using this magnetic carrier, the density of a solid black portion can be uniformalized without reduction of the resolving power.
However, although this known developer is capable of increasing the image density of a solid image portion, in the reproduction of multiple fine lines, the line width is not constant among the respective lines and lacking of the top end or rear end is caused, and the general image quality is still unsatisfactory.
The characteristics of heretofore proposed magnetic carriers are defined by static conditions such as resistivity, particle size, shape and dielectric constant, and selection of a magnetic carrier or adjustment of the amount of a coating resin based on such static conditions is not defined by factors under dynamic conditions in an actual copying machine. Namely, the characteristics in the state of dynamic constant between the magnetic brush of the developer on the developing sleeve and the surface of the photosensitive material are not defined. Accordingly, sufficient correspondence of these characteristics to the actual developing conditions cannot be found.
In view of this circumstance, in the present invention, the amount of a resin coated on the carrier is determined based on the current value. However, if this characteristic alone is specified, though the above-mentioned carrier dragging or reduction of the image density is not caused, letter thinning or fogging is sometimes caused and this adjustment of the amount coated of the resin is still insufficient.
An organic photosensitive material which has a good processability and is advantageous in the manufacturing cost and has a large freedom of the design of functions is recently used as the photosensitive material for the electrophotography. The organic photosensitive material includes a negatively chargeable type and a positively chargeable type. Since the negatively chargeable type often induces contamination of the copying environment, use of the positively chargeable photosensitive material is now expected.
In this positively chargeable photosensitive material, however, the residual voltage is apt to become larger than in the conventional Se type photosensitive material, and therefore, in the case where the positively chargeable photosensitive material is used, the bias voltage should be maintained at a level higher than in the conventional technique. Elevation of the bias voltage increases the charge repulsion between the magnetic carrier and the developing sleeve. Accordingly, carrier dragging is often caused. Therefore, at the development of the positively chargeable photosensitive material, prevention of carrier dragging and improvement of the image density are required.